Method of manufacturing positive electrode material for lithium ion secondary battery and positive electrode material for lithium ion secondary battery

ABSTRACT

A method of manufacturing a positive electrode material for lithium ion secondary battery includes the following (α) and (β): (α) a positive electrode active material is prepared; and (β) the positive electrode material for lithium ion secondary battery is manufactured by forming a coat on at least a portion of a surface of the positive electrode active material. The coat is formed to satisfy the following (1) to (3): (1) the coat includes a lithium ion conductor and a ferroelectric substance; (2) the ferroelectric substance is dispersed in the lithium ion conductor; and (3) the lithium ion conductor is interposed at least partially between the positive electrode active material and the ferroelectric substance.

This nonprovisional application is based on Japanese Patent ApplicationNo. 2017-041771 filed on Mar. 6, 2017, with the Japan Patent Office, theentire contents of which are hereby incorporated by reference.

BACKGROUND Field

The present disclosure relates to a method of manufacturing a positiveelectrode material for lithium ion secondary battery and the positiveelectrode material for lithium ion secondary battery.

Description of the Background Art

Japanese Patent Laying-Open No. 2014-116129 discloses that a positiveelectrode active material is coated with a ferroelectric substance.

SUMMARY

Japanese Patent Laying-Open No. 2014-116129 is expected to provide aneffect of reducing battery resistance by coating the positive electrodeactive material with the ferroelectric substance (for example, bariumtitanate or the like). However, generally, it is considered that theferroelectric substance is poor in lithium (Li) ion conductivity. Hence,desertion and insertion of Li ions are considered to be blocked in thepositive electrode active material at its region directly coated withthe ferroelectric substance. That is, it is considered that an effectivereaction area is decreased on the surface of the positive electrodeactive material. This is considered to decrease the effect of reducingthe battery resistance.

An object of the present disclosure is to provide a positive electrodematerial for lithium ion secondary battery to allow for a large effectof reducing battery resistance.

Hereinafter, the technical configuration and function and effect of thepresent disclosure will be described. However, the mechanism of thefunction of the present disclosure includes presumption. The scope ofclaims should not be limited depending on whether the presumed mechanismis correct or incorrect.

[1] A method of manufacturing a positive electrode material for lithiumion secondary battery includes the following (α) and (β):

(α) a positive electrode active material is prepared; and

(β) the positive electrode material for lithium ion secondary battery ismanufactured by forming a coat on at least a portion of a surface of thepositive electrode active material.

The coat is formed to satisfy the following (1) to (3):

(1) the coat includes a lithium ion conductor and a ferroelectricsubstance;

(2) the ferroelectric substance is dispersed in the lithium ionconductor; and

(3) the lithium ion conductor is interposed at least partially betweenthe positive electrode active material and the ferroelectric substance.

The forming of the coat includes the following (β1) and (β2):

(β1) a first layer is formed on the surface of the positive electrodeactive material; and

(β2) a second layer is layered on the first layer.

The first layer includes the lithium ion conductor.

The second layer includes the lithium ion conductor and theferroelectric substance.

The coat is formed from the first layer and the second layer.

It is considered that the coat of the present disclosure is a compositematerial of the Li ion conductor and the ferroelectric substance. In themanufacturing method of the present disclosure, the coat is formed suchthat the Li ion conductor is at least partially interposed between thepositive electrode active material and the ferroelectric substance.Therefore, it is expected to suppress the effective reaction area frombeing deceased due to the surface of the positive electrode activematerial being directly coated with the ferroelectric substance.

Further, in the manufacturing method of the present disclosure, the coatis formed such that the ferroelectric substance is dispersed in the Liion conductor. Accordingly, it is expected that the Li ion diffusionpath by the Li ion conductor is formed between the surface of the coatand the surface of the positive electrode active material. Further, itis expected to facilitate diffusion of Li ions by dielectricpolarization (alignment of electric dipole) of the ferroelectricsubstance in the diffusion path (Li ion conductor). With the synergiceffect of the above-described functions, it is expected to increase theeffect of reducing battery resistance.

The coat of the present disclosure can be formed by layering the secondlayer including both the Li ion conductor and the ferroelectricsubstance on the first layer including the Li-ion conductor.

[2] The lithium ion conductor may include a compound or a solidsolution. The compound or the solid solution may include: (i) Li; (ii)at least one selected from a group consisting of P, Al, Si, Zr, Ti, Zn,Nb, Ta, and W; and (iii) O or S. The compound or solid solution havingsuch a composition is expected to exhibit Li ion conductivity.

[3] The ferroelectric substance may include a perovskite type oxide.

The perovskite type oxide may be represented by the following formula(I):

ABO₃   (I)

where A is different from B,

A includes at least one of Ba and Sr, and

B includes Ti. The perovskite type oxide having such a composition isexpected to exhibit ferroelectricity.

[4] The positive electrode active material may include a lamellar rocksalt type oxide. The lamellar rock salt type oxide includes at least Li,Ni, Co, Mn, and O. This lamellar rock salt type oxide is expected tohave a large specific capacity (capacity per unit mass) and have a lowresistance.

[5] A positive electrode material for lithium ion secondary batteryincludes: a positive electrode active material; and a coat. The coat isformed on at least a portion of a surface of the positive electrodeactive material. The coat includes a lithium ion conductor and aferroelectric substance. The ferroelectric substance is dispersed in thelithium ion conductor. The lithium ion conductor is interposed at leastpartially between the positive electrode active material and theferroelectric substance. This positive electrode material for lithiumion secondary battery is expected to attain a large effect of reducingbattery resistance.

[6] The lithium ion conductor may include a compound or a solidsolution. The compound or the solid solution may include: (i) Li; (ii)at least one selected from a group consisting of P, Al, Si, Zr, Ti, Zn,Nb, Ta, and W; and (iii) O or S. The compound or solid solution havingsuch a composition is expected to exhibit Li ion conductivity.

[7] The ferroelectric substance may include a perovskite type oxide.

The perovskite type oxide may be represented by the following formula(I):

ABO₃   (I)

where A is different from B,

A includes at least one of Ba and Sr, and

B includes Ti. The perovskite type oxide having such a composition isexpected to exhibit ferroelectricity.

[8] The positive electrode active material may include a lamellar rocksalt type oxide. The lamellar rock salt type oxide includes at least Li,Ni, Co, Mn, and O. This lamellar rock salt type oxide has a largespecific capacity and is expected to attain low resistance.

The foregoing and other objects, features, aspects and advantages of thepresent disclosure will become more apparent from the following detaileddescription of the present disclosure when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flowchart schematically showing a method of manufacturing apositive electrode material for lithium ion secondary battery accordingto an embodiment of the present disclosure.

FIG. 2 is a flowchart showing formation of a coat.

FIG. 3 is a conceptual view showing the positive electrode material forlithium ion secondary battery according to the embodiment of the presentdisclosure.

FIG. 4 is a conceptual view showing a positive electrode material forlithium ion secondary battery according to a reference embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, an embodiment (hereinafter, referred to as “the presentembodiment”) of the present disclosure will be described. However, thedescription below is not intended to limit the scope of claims. In thedescription below, the term “positive electrode material for lithium ionsecondary battery” may be simply described as “positive electrodematerial.”<Method of Manufacturing Positive Electrode Material forLithium Ion Secondary Battery>

FIG. 1 is a flowchart schematically showing a method of manufacturing apositive electrode material for lithium ion secondary battery accordingto an embodiment of the present disclosure. The manufacturing methodincludes “(α) preparation of the positive electrode active material” and“(β) formation of a coat”. Hereinafter, a sequence of the manufacturingmethod will be described.

<<(α) Preparation of Positive Electrode Active Material>>

The manufacturing method of the present embodiment includes preparingthe positive electrode active material. The positive electrode activematerial permits Li ions to be inserted thereinto and deserted therefromelectrochemically. Here, the positive electrode active material may bepurchased or may be synthesized. The positive electrode active materialmay be a particulate matter, for example. Details of the positiveelectrode active material will be described later.

<<(β) Formation of Coat>>

The manufacturing method of the present embodiment includesmanufacturing the positive electrode material by forming the coat on atleast a portion of a surface of the positive electrode active material.

The coat of the present embodiment is formed to satisfy the following(1) to (3):

(1) the coat includes a Li ion conductor and a ferroelectric substance;

(2) the ferroelectric substance is dispersed in the Li ion conductor;and

(3) the Li ion conductor is interposed at least partially between thepositive electrode active material and the ferroelectric substance.

A method of forming the coat should not be limited in particular as longas a coat satisfying the conditions (1) to (3) above can be formed. Thecoat can be formed by a hydrothermal synthesis method, a sol-gel method,an atomic layer deposition (ALD) method, a chemical vapor deposition(CVD) method, or the like, for example.

FIG. 2 is a flowchart showing formation of the coat. As shown in FIG. 2,the “(β) formation of the coat” includes “(β1) formation of a firstlayer” and “(β2) formation of a second layer”. Here, by way of example,the following describes an embodiment in which the coat (the first layerand the second layer) is formed by the sol gel method.

<<(β1) Formation of First Layer>>

The forming of the coat includes forming the first layer on a surface ofthe positive electrode active material. The first layer is formed toinclude the Li ion conductor. In view of an effective reaction area, thefirst layer is desirably formed to include substantially only the Li ionconductor.

For example, a precursor solution of the Li ion conductor is preparedfirst. For example, lithium alkoxide and tungsten alkoxide are dissolvedin a solvent. Accordingly, the precursor solution of the Li ionconductor (in this example, Li₂WO₄) is prepared. This precursor solutionwill be referred to as “first layer precursor solution” for the sake ofconvenience.

The positive electrode active material is agitated in the first layerprecursor solution. Accordingly, the precursor to be the first layer isadhered on the surface of the positive electrode active material. Thesolvent is removed by drying. Accordingly, a dry solid matter iscollected. The dry solid matter is heated to 300 to 1000° C., forexample. The heating temperature can be appropriately changed accordingto a type of the Li ion conductor. By the heating, the Li ion conductoris generated from the precursor. That is, the first layer is formed onthe surface of the positive electrode active material. The first layerincludes the Li ion conductor. It is considered that the Li ionconductor included in the first layer is interposed at least partiallybetween the positive electrode active material and a ferroelectricsubstance described below. Details of the Li ion conductor will bedescribed later.

<<(β2) Formation of Second Layer>>

The forming of the coat includes layering the second layer on the firstlayer. The second layer is formed to include the Li ion conductor andthe ferroelectric substance. The coat is formed from the first layer andthe second layer. It should he noted that in the final coat, a boundarybetween the first layer and the second layer may be unable to beconfirmed because the first layer is integrated with the second layercompound.

For example, a precursor solution of the ferroelectric substance isprepared first. For example, barium alkoxide and titanium alkoxide aredissolved in a solvent. Accordingly, a precursor solution of theferroelectric substance (in this example, BaTiO₃) is prepared. Next, theprecursor solution of the ferroelectric substance is mixed with theprecursor solution of the Li ion conductor. Accordingly, a solution isprepared which includes both the precursor of the Li ion conductor andthe precursor of the ferroelectric substance. Hereinafter, this solutionwill be referred to as “second layer precursor solution” for the sake ofconvenience.

It should be noted that the Li ion conductor of the first layer may bethe same as or different from the Li ion conductor of the second layer.

In the second layer precursor solution, the positive electrode activematerial including the first layer is agitated. Accordingly, theprecursor to be the second layer is adhered to the surface of the firstlayer. The solvent is removed by drying. Accordingly, a dry solid matteris collected. The dry solid matter is heated to 300 to 1000° C., forexample. The heating temperature can be appropriately changed accordingto types of the Li ion conductor and ferroelectric substance. The Li ionconductor and the ferroelectric substance are generated from therespective precursors by the heating. Accordingly, the second layer isformed. That is, the second layer is layered on the first layer. Thecoat is formed from the first layer and the second layer.

The second layer includes the Li ion conductor and the ferroelectricsubstance. It is considered that the ferroelectric substance isdispersed in the Li ion conductor. Details of the ferroelectricsubstance will be described later.

One set of the “(β1) formation of the first layer” and the “(β2)formation of the second layer” may be repeated multiple times. In themanner described above, the positive electrode material for lithium ionsecondary battery in the present embodiment can be manufactured.

<Positive Electrode Material for Lithium Ion Secondary Battery>

FIG. 3 is a conceptual view showing the positive electrode material forlithium ion secondary battery according to the embodiment of the presentdisclosure. A positive electrode material 100 includes a positiveelectrode active material 10 and a coat 20. Coat 20 is formed on atleast a portion of the surface of positive electrode active material 10.Coat 20 includes a Li ion conductor 21 and a ferroelectric substance 22.Ferroelectric substance 22 is dispersed in Li ion conductor 21. Li ionconductor 21 is interposed at least partially between positive electrodeactive material 10 and ferroelectric substance 22.

An arrangement of positive electrode active material 10, Li ionconductor 21, and ferroelectric substance 22 can be confirmed byanalyzing a cross sectional sample of positive electrode material 100using a scanning transmission electron microscope-energy dispersiveX-ray spectrometry (STEM-EDX) method or the like, for example. The crosssectional sample can be prepared by focused ion beam (FIB) or the like,for example.

In positive electrode material 100, Li ion conductor 21 is interposed atleast partially between positive electrode active material 10 andferroelectric substance 22. Accordingly, it is expected to suppress theeffective reaction area from being decreased by ferroelectric substance22 directly coating the surface of positive electrode active material10. In view of the effective reaction area, Li ion conductor 21 isdesirably interposed entirely between positive electrode active material10 and ferroelectric substance 22.

Ferroelectric substance 22 is dispersed in Li ion conductor 21.Accordingly, Li ion conductor 21 is expected to form a Li ion (Li⁺)diffusion path between the surface of coat 20 and the surface ofpositive electrode active material 10. Further, it is expected tofacilitate diffusion of Li ions by dielectric polarization (alignment ofelectric dipole) of ferroelectric substance 22 in the diffusion path (Liion conductor 21). With the synergic effect of the above-describedfunctions, it is expected to increase the effect of reducing batteryresistance.

FIG. 4 is a conceptual view showing a positive electrode material forlithium ion secondary battery according to a reference embodiment. In apositive electrode material 200, a ferroelectric substance 22 isdirectly adhered to a surface of a positive electrode active material10. Due to dielectric polarization of ferroelectric substance 22, it isconsidered to facilitate insertion/desertion of Li ions into/frompositive electrode active material 10 in the vicinity of ferroelectricsubstance 22. However, generally, it is considered that ferroelectricsubstance 22 is poor in Li ion conductivity. Therefore, it is consideredthat insertion and desertion of Li ions are blocked in positiveelectrode active material 10 at the region directly coated withferroelectric substance 22. That is, it is considered that the effectivereaction area is decreased in the surface of positive electrode activematerial 10. Accordingly, it is considered that the effect of reducingbattery resistance is decreased.

The following describes each of the materials in positive electrodematerial 100 of the present embodiment.

<<Positive Electrode Active Material>>

Positive electrode active material 10 permits Li ions to be insertedthereinto and deserted therefrom electrochemically. Positive electrodeactive material 10 may be a particulate matter, for example. Forexample, positive electrode active material 10 may be secondaryparticles resulting from gathered primary particles. Positive electrodeactive material 10 may have a uniform composition or may have acomposition differing locally. For example, the secondary particles maybe formed from two or more types of primary particles having differentcompositions.

Positive electrode active material 10 (secondary particles) may have anaverage particle size of 1 to 30 μm, may have an average particle sizeof 1 to 20 μm, or may have an average particle size of 5 to 15 μm, forexample. The “average particle size” in the present specificationrepresents the size of particles at an integrated value of 50% from thefinest particle in volume-based particle size distribution measured by alaser diffraction scattering method.

Positive electrode active material 10 should not be limited particularlyas long as positive electrode active material 10 permits Li ions to beinserted thereinto and deserted therefrom electrochemically. Examples ofthe positive electrode active material may include a lamellar rock salttype oxide, a spinel type oxide, an olivine type compound, and the like.The crystal structure of positive electrode active material 10 can beidentified by an X-ray diffraction (XRD) method, an electron diffractionmethod, or the like, for example. The composition of positive electrodeactive material 10 can be measured by the EDX method or the like, forexample.

Examples of the lamellar rock salt type oxide include LiNiO₂, LiCoO₂,LiMnO₂, and the like. The lamellar rock salt type oxide may include atleast Li, Ni (nickel), Co (cobalt), Mn (manganese), and O (oxygen). Thislamellar rock salt type oxide is also referred to as “ternary lamellarrock salt type oxide”. The ternary lamellar rock salt type oxide has alarge specific capacity and is expected to attain low resistance.

In the ternary lamellar rock salt type oxide, Ni, Co, and Mn may bepartially replaced with a different element. Accordingly, it is expectedto improve cycle durability, for example. This is presumably because thepartial replacement of the elements leads to a stabilized crystalstructure. In the ternary lamellar rock salt type oxide, part of oxygenmay be replaced with a halogen element. Accordingly, it is expected toreduce resistance, for example. This is presumably because theconcentration of the Li ions is increased near positive electrode activematerial 10 due to the halogen element, which has highelectronegativity, for example.

For example, the ternary lamellar rock salt type oxide may berepresented by the following formula (II):

Li_((1+a))Ni_(b)Co_(c)Mn_((1-b-c))M_(d)O_((2-e))X_(e)   (II)

where

a, b, c, d, and e satisfy 0≤a≤0.7, 0.1≤b≤0.9, 0.1≤c≤0.4, 0≤d≤0.6, and0≤e≤0.5,

M is at least one selected from a group consisting of Zr (zirconium), Mo(molybdenum), W (tungsten), Mg (magnesium), Ca (calcium), Na (sodium),Fe Cr (chromium), Zn (zinc), Si (silicon), Sn (tin) and Al (aluminum),and

X is at least one selected from a group consisting of F (fluorine), Cl(chlorine), and Br (bromine).

Examples of the spinel type oxide include LiMn₂O₄, LiNi_(0.5)Mn_(1.5)O₄,and the like. Examples of the olivine type compound include LiFePO₄,LiMnPO₄, and the like.

<<Coat>>

Coat 20 is formed at least partially on the surface of positiveelectrode active material 10. Coat 20 may be formed entirely on thesurface of positive electrode active material 10, or may be formedpartially on the surface of positive electrode active material 10. Sincecoat 20 is formed at least partially on the surface of positiveelectrode active material 10, it is expected to obtain the effect ofreducing battery resistance. Coat 20 includes Li ion conductor 21 andferroelectric substance 22.

<<Li Ion Conductor>>

Li ion conductor 21 of the present embodiment represents a solid thatdiffuses Li ions therein. Coat 20 may solely include one type of Li ionconductor 21 or may include two or more types of Li ion conductors 21.

The composition of Li ion conductor 21 may be measured by an EDX methodor the like, for example. Li ion conductor 21 may also include acompound or a solid solution. The compound or the solid solution mayinclude: (i) Li; (ii) at least one from a group consisting of P(phosphorus), Al, Si, Zr, Ti (titanium), Zn, Nb (niobium), Ta(tantalum), and W; (iii) O or S (sulfur), for example. It is expectedthat the compound or solid solution having such a composition exhibitsLi ion conductivity.

The elements) in (ii) above may be at least one selected from a groupconsisting of P, Al, Si, Zr, Ti, Zn, Nb, Ta, W, Mg, Mo, and a rare earthelement.

Li ion conductor 21 may be at least one selected from a group consistingof Li₂WO₄, Li₃PO₄, LiTaO₃, Li₄SiO₄, Li₃PO₄—Li₄SiO₄, Li₂ZrO₃, Li₄Ti₅O₁₂,LiNbO₃, Li₂S—P₂S₅, Li₃Zn_(0.5)Nb₂O₇, and Li₅Al₂O₃, for example. Here,“Li₃PO₄—Li₄SiO₄” represents a solid solution of Li₃PO₄ and Li₄SiO₄.“Li₂S—P₂S₅” represents a solid solution of Li₂S and P₂S₅.

When Li ion conductor 21 is Li₂WO₄, Li₃PO₄, or Li₂ZrO₃, it is expectedto increase the effect of reducing battery resistance. Therefore, Li ionconductor 21 may be at least one selected from a group consisting ofLi₂WO₄, Li₃PO₄, and Li₂ZrO₃.

<<Ferroelectric Substance>>

Ferroelectric substance 22 of the present embodiment has spontaneouspolarization even when there is no external electric field, and exhibitsa crystal in which a direction of polarization is reversed according toa direction of electric field. The crystal structure of ferroelectricsubstance 22 can be identified by the XRD method, the electrondiffraction method, or the like, for example. The composition offerroelectric substance 22 may be measured by the EDX method or thelike, for example.

Ferroelectric substance 22 may be a perovskite type oxide, for example.

The perovskite type oxide may be represented by the following formula(I):

ABO₃   (I)

where A is different from B,

A includes at least one of Ba (barium) and Sr (strontium), and

B includes Ti.

It is expected that the perovskite type oxide having such a compositionexhibits ferroelectricity. The perovskite type oxide may be at least oneof BaTiO₃ and SrTiO₃.

In the formula (I), A (A site element) may be at least one selected froma group consisting of Pb (lead), Ba, Sr, Bi (bismuth), Li, Na, Ca, Cd(cadmium), Mg, K (potassium), and a lanthanoid element, for example.

In the formula (I), B (B site element) may be at least one selected froma group consisting of Ti, Zr, V (vanadium), Nb, Ta, Sb (antimony), Cr,Mo, W, Mn, Mg, Sc (scandium), Co, Cu (copper), In (indium), Sn, Ga(gallium), Zn, Cd, Fe, Ni, Hf (hafnium), and Al, for example.

<<Application>>

It is expected that positive electrode material 100 attains a largeeffect of reducing battery resistance. Therefore, it is expected that alithium ion secondary battery including positive electrode material 100exhibits a high output. Examples of applications requiring such a highoutput include power supplies for hybrid vehicles (HV), plug-in hybridvehicles (PHV), and electric vehicles (EV). However, the applications ofthe lithium ion secondary battery including positive electrode material100 should not be limited to such in-vehicle applications. The lithiumion secondary battery including positive electrode material 100 isapplicable to any applications.

EXAMPLES

Hereinafter, examples of the present disclosure will be described. Theexamples below, however, do not limit the scope of claims.

Comparative Example 1

1. Preparation of Positive Electrode Active Material

Ni sulfate, Co sulfate and Mn sulfate were dissolved in pure water suchthat a molar ratio of Ni, Co and Mn became Ni:Co:Mn=1:1:1. Accordingly,a sulfate aqueous solution was obtained. A sodium hydroxide (NaOH)aqueous solution was dropped to the sulfate aqueous solution.Accordingly, a precursor (coprecipitation hydroxide) of the positiveelectrode active material was generated. The precursor was cleaned bypure water. The precursor having been cleaned was dried. The precursorhaving been dried was mixed with lithium carbonate (Li₂CO₃).Accordingly, a mixture was obtained. The mixture was heated at 900° C.for 15 hours. Accordingly, a calcinated material was obtained. Thecalcinated material was pulverized by a ball mill.

In this way, a positive electrode active material (LiNi_(1/3)Co_(1/3)Mn_(1/3)O₂) was prepared. This positive electrode activematerial is a lamellar rock salt type oxide (ternary lamellar rock salttype oxide), and includes Li, Ni, Co, Mn, and O. This positive electrodeactive material had an average particle size of 10 μm. Hereinafter, thispositive electrode active material may be abbreviated as “NCM”.

Preparation of Positive Electrode Composite Material Paste

The following materials were prepared.

Conductive material: acetylene black

Binder: polyvinylidene fluoride

Solvent: N-methyl-2-pyrrolidone

A planetary mixer was employed to mix the positive electrode activematerial obtained above, a conductive material, a binder, and a solvent.Accordingly, a positive electrode composite material paste was prepared.The solid content composition of the positive electrode compositematerial paste is as follows: “the positive electrode activematerial:the conductive material:the binder=84:12:4” in mass ratio. Thesolid content ratio of the positive electrode composite material pastewas 56 mass %.

3. Manufacturing of Positive Electrode Plate

An Al foil in the form of a band was prepared. A die coater was employedto apply the obtained positive electrode composite material paste ontothe surface of the Al foil (both the front and rear surfaces) and thepositive electrode composite material paste was dried. Accordingly, thepositive active material layer was formed on the surface of the Al foil.A roller was employed to roll the positive active material layer and theAl foil. In this way, a positive electrode plate in the form of a bandwas manufactured.

4. Manufacturing of Lithium Ion Secondary Battery

A negative electrode plate in the form of a band and a separator in theform of a band were prepared. The positive electrode plate, theseparator, and the negative electrode plate were layered such that thepositive electrode plate and the negative electrode plate face eachother with the separator being interposed therebetween, and were thenwound into a spiral form. Accordingly, an electrode group wasconstructed. Terminals were connected to the positive electrode plateand the negative electrode plate, respectively. The electrode group wasstored in a battery case. An electrolyte solution was injected into thebattery case. The battery case was sealed. In this way, a lithium ionsecondary battery was manufactured. Hereinafter, the lithium ionsecondary batter may be simply described as “battery”.

Example 1

1-1. (α) Preparation of Positive Electrode Active Material

In the same procedure as that in Comparative Example 1, the positiveelectrode active material (NCM) was prepared.

1-2. (β) Formation of Coat

1-2-1. (β1) Formation of First Layer

Lithium alkoxide and tungsten alkoxide were dissolved in a solvent.Accordingly, a first layer precursor solution was prepared. This firstlayer precursor solution includes a precursor of Li₂WO₄. The positiveelectrode active material was introduced into the first layer precursorsolution such that Li₂WO₄ became 0.25 mol % with respect to the positiveelectrode active material.

The positive electrode active material was agitated in the first layerprecursor solution. Accordingly, a precursor to be the first layer isadhered on the surface of the positive electrode active material. Thesolvent was removed by drying. A dry solid matter was collected. The drysolid matter was heated at 700° C. By the heating, Li₂WO₄ was generatedfrom the precursor, thereby forming a first layer.

1-2-2. (β2) Formation of Second Layer

Barium alkoxide and titanium alkoxide were dissolved in a solvent.Accordingly, a precursor solution of the ferroelectric substance wasprepared. The precursor solution of the ferroelectric substance and thefirst layer precursor solution obtained above were mixed. Accordingly, asecond layer precursor solution was prepared. This second layerprecursor solution includes both a precursor of the Li ion conductor(Li₂WO₄) and a precursor of the ferroelectric substance (BaTiO₃).

The positive electrode active material including the first layerobtained above was introduced into the second layer precursor solutionsuch that Li₂WO₄ became 0.25 mol % and BaTiO₃ became 0.5 mol % withrespect to the positive electrode active material including the firstlayer.

In the second layer precursor solution, the positive electrode activematerial including the first layer was agitated. Accordingly, theprecursor to be the second layer is adhered onto the surface of thefirst layer. The solvent was removed by drying. Accordingly, a dry solidmatter was collected. The dry solid matter was heated at 700° C. By theheating, Li₂WO₄ and BaTiO₃ were generated from the precursorsrespectively. Accordingly, a second layer was formed. That is, thesecond layer was layered on the first layer. The coat was formed fromthe first layer and the second layer. In this way, the positiveelectrode material according to Example 1 was manufactured. In Example1, each of Li₂WO₄ and BaTiO₃ was 0.5 mol % with respect to the positiveelectrode active material.

As the positive electrode active material, the positive electrodematerial according to Example 1 was used, and “2. Preparation ofPositive Electrode Composite Material Paste”, “3. Manufacturing ofPositive Electrode Plate” and “4. Manufacturing of Lithium Ion SecondaryBattery” were sequentially performed in the same manner as inComparative Example 1, thereby manufacturing a battery.

Comparative Example 2

In the precursor solution of the ferroelectric substance, the positiveelectrode active material was agitated. Accordingly, a precursor of theferroelectric substance was adhered onto the surface of the positiveelectrode active material. The solvent was removed by drying.Accordingly, a dry solid matter was collected. The dry solid matter washeated at 700° C. By the heating, BaTiO₃ was generated from theprecursor. In this way, the positive electrode material according toComparative Example 2 was manufactured. In Comparative Example 2, BaTiO₃was adjusted to 1.0 mol % with respect to the positive electrode activematerial.

As the positive electrode active material, the positive electrodematerial according to Comparative Example 2 was used, and “2.Preparation of Positive Electrode Composite Material Paste”, “3.Manufacturing of Positive Electrode Plate” and “4. Manufacturing ofLithium Ion Secondary Battery” were sequentially performed in the samemanner as in Comparative Example 1, thereby manufacturing a battery.

Comparative Example 3

In the precursor solution of the Li ion conductor, the positiveelectrode active material was agitated. Accordingly, the precursor ofthe Li ion conductor was adhered on the surface of the positiveelectrode active material. The solvent was removed by drying.Accordingly, a dry solid matter was collected. The dry solid matter washeated at 700° C. By the heating, was generated from the precursor. Inthis way, the positive electrode material according to ComparativeExample 3 was manufactured. In Comparative Example 3, Li₂WO₄ wasadjusted to 1.0 mol % with respect to the positive electrode activematerial.

As the positive electrode active material, the positive electrodematerial according to Comparative Example 3 was used, and “2.Preparation of Positive Electrode Composite Material Paste”, “3.Manufacturing of Positive Electrode Plate” and “4. Manufacturing ofLithium Ion Secondary Battery” were sequentially performed in the samemanner as in Comparative Example 1, thereby manufacturing a battery.

Comparative Example 4

In the precursor solution of the ferroelectric substance, the positiveelectrode active material was agitated. Accordingly, a precursor of theferroelectric substance was adhered onto the surface of the positiveelectrode active material. The solvent was removed by drying.Accordingly, a dry solid matter was collected. The dry solid matter washeated at 700° C. By the heating, BaTiO₃ was generated from theprecursor, thereby forming the first layer. This first layer includesonly the ferroelectric substance.

Next, the positive electrode active material including the first layerwas agitated in the precursor solution of the Li ion conductor.Accordingly, the precursor to be the second layer was adhered to thesurface of the first layer. The solvent was removed by drying.Accordingly, a dry solid matter was collected. The dry solid matter washeated at 700° C. By the heating, Li₂WO₄ was generated from theprecursor. Accordingly, a second layer was formed. That is, the secondlayer was layered on the first layer. This second layer includes onlythe Li ion conductor. The coat was formed from the first layer and thesecond layer. In this way, the positive electrode material according toComparative Example 4 was manufactured. In Comparative Example 4, eachof Li₂WO₄ and BaTiO₃ was 0.5 mol % with respect to the positiveelectrode active material.

As the positive electrode active material, the positive electrodematerial according to Comparative Example 4 was used, and “2.Preparation of Positive Electrode Composite Material Paste”, “3.Manufacturing of Positive Electrode Plate” and “4. Manufacturing ofLithium Ion Secondary Battery” were sequentially performed in the samemanner as in Comparative Example 1, thereby manufacturing a battery.

Examples 2 to 11

Each of positive electrode materials according to Examples 2 to 11 wasmanufactured in the same manner as in Example except that first andsecond layer precursor solutions were prepared to generate a Li ionconductor shown in Table 1 below, thereby manufacturing a battery.

Examples 12 to 22

Each of positive electrode materials according to Examples 12 to 22 wasmanufactured in the same manner as in Examples 2 to 11 except that asecond layer precursor solution was prepared to generate SrTiO₃ as aferroelectric substance, thereby manufacturing a battery.

<Evaluation>

1. Activation of Battery and Measurement of Initial Capacity

At 25° C., the battery was fully charged by below-described constantcurrent-constant voltage mode charging (CCCV charging). Next, thebattery was discharged by below-described constant current modedischarging (CC discharging). The discharge capacity on this occasionwas assumed as an initial capacity. It should be noted that “1C”represents current with which the full charge capacity is discharged in1 hour.

CCCV charging: CC current=1/3C; CV voltage=4.2V; and cutoffcurrent=1/50C

CC discharging: current=1/3C; and end voltage=3.0V

2. Evaluation of Battery Resistance

The SOC (State Of Charge) of the battery was adjusted to 56%. On thisoccasion, the open circuit voltage of the battery was 3.7V. Under a 25°C. environment, the battery was discharged until voltage between theterminals became 3.0V. The discharging was performed by way of the CCdischarging. An amount of decrease in the voltage between the terminals5 seconds after starting the discharge was measured. The amount ofdecrease in the voltage between the terminals was divided by thedischarging current, thereby calculating battery resistance. Results areshown in the column “Battery Resistance” of Table 1 below. The valueshown herein is a value obtained by dividing the battery resistance ofeach example by the battery resistance of Comparative Example 1. It isindicated that as the value is smaller, the effect of reducing batteryresistance is larger.

3. Evaluation of Cycle Durability

Under a 60° C. environment, 200 cycles were performed, wherein one cyclerepresents one set of the following constant current charging (CCcharging) and CC discharging.

CC charging: current=2C and end voltage=4.3 V

CC discharging: current=2C and end voltage=3.0 V

After the 200 cycles, a post-cycle capacity was measured under the sameconditions as those for the initial capacity. By dividing the post-cyclecapacity by the initial capacity, a capacity maintenance ratio wascalculated. Results are shown in the column “Capacity Maintenance Ratio”in Table 1 below. It is indicated that as the capacity maintenance ratiois higher, the cycle durability is more excellent.

TABLE 1 List of Examples and Comparative Examples Positive ElectrodeMaterial for Lithium Ion Secondary Battery Battery Performance CoatCapacity Positive First Layer Second Layer Battery Maintenance ElectrodeFerroelectric Li Ion Ferroeleetric Li Ion Resistance Ratio ActiveMaterial Substance Conductor Substance Conductor [—] [%] ComparativeExample 1 NCM — — — — 1 60 Comparative Example 2 NCM BaTiO₃ — — — 0.8074 Comparative Example 3 NCM — Li₂WO₄ — — 0.81 76 Comparative Example 4NCM BaTiO₃ — — Li₂WO₄ 0.90 66 Example 1 NCM — Li₂WO₄ BaTiO₃ Li₂WO₄ 0.5093 Example 2 NCM — Li₃PO₄ BaTiO₃ Li₃PO₄ 0.54 91 Example 3 NCM — LiTaO₃BaTiO₃ LiTaO₃ 0.63 92 Example 4 NCM — Li₄SiO₄ BaTiO₃ Li₄SiO₄ 0.62 90Example 5 NCM — Li₃PO₄—Li₄SiO₄ BaTiO₃ Li₃PO₄—Li₄SiO₄ 0.58 91 Example 6NCM — Li₂ZrO₃ BaTiO₃ Li₂ZrO₃ 0.50 93 Example 7 NCM — Li₄Ti₅O₁₂ BaTiO₃Li₄Ti₅O₁₂ 0.68 90 Example 8 NCM — LiNbO₃ BaTiO₃ LiNbO₃ 0.66 94 Example 9NCM — Li₂S—P₂S₅ BaTiO₃ Li₂S—P₂S₅ 0.60 89 Example 10 NCM —Li₃Zn_(0.5)Nb₂O₇ BaTiO₃ Li₃Zn_(0.5)Nb₂O₇ 0.70 87 Example 11 NCM —Li₅Al₂O₃ BaTiO₃ Li₅Al₂O₃ 0.68 85 Example 12 NCM — Li₂WO₄ SrTiO₃ Li₂WO₄0.55 90 Example 13 NCM — Li₃PO₄ SrTiO₃ Li₃PO₄ 0.57 93 Example 14 NCM —LiTaO₃ SrTiO₃ LiTaO₃ 0.64 89 Example 15 NCM — Li₄SiO₄ SrTiO₃ Li₄SiO₄0.63 94 Example 16 NCM — Li₃PO₄—Li₄SiO₄ SrTiO₃ Li₃PO₄—Li₄SiO₄ 0.62 94Example 17 NCM — Li₂ZrO₃ SrTiO₃ Li₂ZrO₃ 0.54 89 Example 18 NCM —Li₄Ti₅O₁₂ SrTiO₃ Li₄Ti₅O₁₂ 0.67 89 Example 19 NCM — LiNbO₃ SrTiO₃ LiNbO₃0.69 92 Example 20 NCM — Li₂S—P₂S₅ SrTiO₃ Li₂S—P₂S₅ 0.61 90 Example 21NCM — Li₃Zn_(0.5)Nb₂O₇ SrTiO₃ Li₃Zn_(0.5)Nb₂O₇ 0.71 86 Example 22 NCM —Li₅Al₂O₃ SrTiO₃ Li₅Al₂O₃ 0.70 84

<Results>

As shown in Table 1 above, the effect of reducing battery resistance inthe Examples are larger than those in Comparative Examples 2 to 4. Thisis presumably due to the following reason: in each of the Examples, theeffective reaction area is suppressed from being decreased by theferroelectric substance directly coating the surface of the positiveelectrode active material, and a Li ion diffusion path by the Li ionconductor is formed between the surface of the coat and the surface ofthe positive electrode active material.

In each of the Examples, an effect of improving cycle durability is alsorecognized in addition to the effect of reducing battery resistance. Ineach of the coats of the Examples, it is considered that an effect ofsuppressing oxidative decomposition of the electrolyte solution islarger than that in each of the coats of the Comparative Examples.

In Comparative Example 4, the effect of reducing battery resistance issmall. This is presumably because the effective reaction area isdecreased by the ferroelectric substance directly coating the surface ofthe positive electrode active material. Further, since the ferroelectricsubstance is not dispersed in the Li ion conductor, it is considereddifficult to form a Li ion diffusion path by the Li ion conductorbetween the surface of the coat and the surface of the positiveelectrode active material.

In each of the examples in which the Li ion conductors are Li₂WO₄,Li₃PO₄, and Li₂ZrO₃, it is acknowledged that the effect of reducingbattery resistance tends to be large.

Although the embodiments of the present disclosure have been described,the embodiments disclosed herein are illustrative and non-restrictive inany respect. The scope of the present disclosure is defined by the termsof the claims, and is intended to include any modifications within thescope and meaning equivalent to the terms of the claims.

What is claimed is:
 1. A method of manufacturing a positive electrodematerial for lithium ion secondary battery, the method comprising:preparing a positive electrode active material; and manufacturing thepositive electrode material for lithium ion secondary battery by forminga coat on at least a portion of a surface of the positive electrodeactive material, the coat being formed such that the coat includes alithium ion conductor and a ferroelectric substance, the ferroelectricsubstance is dispersed in the lithium ion conductor, and the lithium ionconductor is interposed at least partially between the positiveelectrode active material and the ferroelectric substance, the formingof the coat including forming a first layer on the surface of thepositive electrode active material, and layering a second layer on thefirst layer, the first layer including the lithium ion conductor, thesecond layer including the lithium ion conductor and the ferroelectricsubstance, and the coat being formed from the first layer and the secondlayer.
 2. The method of manufacturing the positive electrode materialfor lithium ion secondary battery according to claim 1, wherein thelithium ion conductor includes a compound or a solid solution, thecompound or the solid solution includes Li, at least one selected from agroup consisting of P, Al, Si, Zr, Ti, Zn, Nb, Ta, and W, and O or S. 3.The method of manufacturing the positive electrode material for lithiumion secondary battery according to claim 1, wherein the ferroelectricsubstance includes a perovskite type oxide, the perovskite type oxide isrepresented by the following formula (I):ABO₃   (I) where A is different from B, A includes at least one of Baand Sr, and B includes Ti.
 4. The method of manufacturing the positiveelectrode material for lithium ion secondary battery according to claim1, wherein the positive electrode active material includes a lamellarrock salt type oxide, and the lamellar rock salt type oxide includes atleast Ni, Co, Mn, and O.
 5. A positive electrode material for lithiumion secondary battery, the positive electrode material comprising: apositive electrode active material; and a coat, the coat being formed onat least a portion of a surface of the positive electrode activematerial, the coat including a lithium ion conductor and a ferroelectricsubstance, the ferroelectric substance being dispersed in the lithiumion conductor, the lithium ion conductor being interposed at leastpartially between the positive electrode active material and theferroelectric substance.
 6. The positive electrode material for lithiumion secondary battery according to claim 5, wherein the lithium ionconductor includes a compound or a solid solution, the compound or thesolid solution includes Li, at least one selected from a groupconsisting of P, Al, Si, Zr, Ti, Zn, Nb, Ta, and W, and O or S.
 7. Thepositive electrode material for lithium ion secondary battery accordingto claim 5, wherein the ferroelectric substance includes a perovskitetype oxide, the perovskite type oxide is represented by the followingformula (I):ABO₃   (I) where A is different from B, A includes at least one of Baand Sr, and B includes Ti.
 8. The positive electrode material forlithium ion secondary battery according to claim 5, wherein the positiveelectrode active material includes a lamellar rock salt type oxide, andthe lamellar rock salt type oxide includes at least Li, Ni, Co, Mn, andO.